This invention relates to welding and it has particular relationship to automatic welding operations in which a work-piece is welded by a series of filler welds carried out automatically in succession. Typical of such automatic welding operations is the welding of casings or cans for large transformers in which successive seam arc welds are produced at the intersections of wall sections at right angles to each other. This application, to an extent, is addressed to the welding of transformer casings in the interest of dealing with concrete practices for the purpose of facilitating the understanding of this invention. It is not intended that the scope of this invention should be limited to such practice. Any use of the principles of this invention is understood to be within the scope of equivalents of any patent which may issue on, or a result of, this application.
The automatic welding of the transformer casings is carried out by arc welding with a stainless steel filler wire which is fed from a spool through a torch to the joint to be welded. The spool and torch are mounted on the movable arm of an electrically controlled and operated robot which manipulates the torch and wire as necessary and controls their operation. Successive seam welds are produced at the joints between wall sections. After the welding of a joint is completed, the casing is rotated automatically and the wire is automatically positioned to weld the succeeding joint, the welding arc is fired and the seam is welded. To position the wire precisely at the joint to be welded, the robot includes a sensor which responds to the electrical conduction between the wire and the work to control the movable arm to position the tip of the filler wire appropriately at the joint. Specifically, an auxiliary potential is impressed between the wire and the work and the sensor senses the resulting current and properly positions the wire at the joint to be welded. It has been found that during the individual welds of a series, the tip of the wire becomes oxidized, increasing the electrical resistance between the wire and the work so that effective electrical conduction between the wire and the work for sensing is precluded. Prior to this invention, it was necessary for the operator to interrupt the automatic operation after a seam was welded and to cut the wire at the oxidized tip. A mechanical shear was provided for this purpose. It was found that this shear became dull after a few operations. In addition, because the shear is an electrical conductor not electrically insulated from the robot, and is grounded, it was necessary to disable the "nozzle touch" safety feature of the robot during the cutting operation. The "nozzle touch" is a protective circuit on the robot that automatically shuts the robot off if the welding nozzle or other part is grounded, i.e., if the nozzle or part strikes a fixture or other grounded structure. In addition, the wire was not cut squarely near the oxidized end so that the new end of the wire was curved instead of being linear and precise positioning of the wire at the joint was not feasible.
It is an object of this invention to overcome the drawbacks and deficiencies of prior practice and the prior art and to provide apparatus and a method for producing a series of welds in work automatically without interruption for manual severing after each weld of the series at the oxidized tip of the filler wire and without disabling the nozzle touch because of grounding. It is also an object of this invention to provide apparatus and a method for automatically severing the oxidized tip of the filler wire in whose use and practice the new tip of the wire, after severing, shall remain linear at its end.